1. Field of the Invention
The present invention relates to an exhaust controller for a spark ignition type, two stroke internal combustion engine. The exhaust controller is capable of adjusting an exhaust timing of the engine depending upon the rotational speed of the engine. Particularly, the present invention relates to an exhaust control valve for the exhaust controller having an improved structure and operation which greatly reduces exhaust leakage past the exhaust control valve.
2. Description of the Background Art
Spark ignition type, two stroke internal combustion engines having an exhaust controller are known in the art. For example, Japanese Patent Publication No. Sho 56-54336 and Japanese Patent Laid-open No. Hei 3-33426 show engines including: an exhaust port and a scavenging port which open into a cylinder hole. The ports are opened and closed by a reciprocating motion of a piston in the cylinder hole. In operation, new gas (unburnt gas) is precompressed in a crank chamber, then fed into the cylinder hole through a scavenging passage and the scavenging port. The new gas is then compressed in the cylinder hole, and ignited by an ignition plug. The burnt gas is exhausted via the exhaust port into an exhaust passage. An exhaust control valve is provided in the exhaust port for reducing the blowoff of the new gas. The exhaust control valve blocks an upper portion of the exhaust port during a low speed rotational state of the engine, and fully opens the exhaust port during a high speed rotational state of the engine.
The internal combustion engine described in Japanese Patent Publication No. Sho 56-54336 uses an exhaust control valve having a form of a double enveloping body. Part of the double enveloping body is cut out in a shape corresponding to a transverse cross-sectional shape of an upper portion of the exhaust passage. In operation, the exhaust valve can be brought into contact with the cylinder hole. For example, during the low speed operational state of the engine, the valve is positioned to partially block the exhaust port (see FIG. 1 of Japanese Patent Publication No. Sho 56-54336). However, due to the structure of the exhaust control valve, when the exhaust control valve is brought into contact with the cylinder hole, exhaust gas tends to leak, with relatively low resistance, through a gap formed between an outer peripheral wall of the piston and an outer peripheral surface of the exhaust control valve. This leakage reduces the exhaust control ability of the exhaust controller during the low speed operational state of the engine.
In the internal combustion engine shown in FIGS. 2 to 4 of Japanese Patent Laid-open No. Hei 3-33426, and as shown in FIGS. 29 to 31 in this specification, an exhaust control valve 01 includes a flat main body 02, a flexible thin plate-like blocking member 03 raised upright from a leading end of the flat main body 02, and an arcuate edge portion 04 of the flat main body 02 extending to a cylinder hole 05. The edge portion 04 conforms to the cylinder hole 05 with a narrow gap kept therebetween.
As shown in FIG. 31, a base portion of the flat main body 02 is pivotably supported about a shaft 07. The shaft is located in an approximately vertical center portion of the exhaust passage 06. A cavity portion 09 is formed in the cylinder block 08. As the flexible thin plate-like blocking member 03 rocks about the shaft 07, it is accepted into the cavity portion 09. In operation, the flexible thin plate-like blocking member 03 is vertically rocked depending on the rotating speed of the engine.
Such a configuration has several shortcomings. One shortcoming occurs when the exhaust control valve 01 is rocked downward in the low speed rotational state of the engine. When the upper edge of a piston 010 is moved up over the upper edge of an exhaust port 011, a gas in the cylinder hole 05 is trapped in a space defined by a cutout portion 012 of the cylinder block 08, blocking member 03, arcuate edge portion 04, and the outer peripheral surface of the piston 010. As a result, the scavenging efficiency and the charging efficiency are reduced and the compression ratio is lowered.
Further, since the exhaust passage 013 is formed in a rectangular shape in cross-section and the base portion of the flat main body 02 is positioned at the approximately vertical center of the exhaust passage 013, the cross-section of the exhaust gas flow passage is rapidly varied. This variance causes a large disturbance in the exhaust flow and thereby increases the exhaust pressure, resulting in a decrease in the output and the efficiency of the engine.
Further, residual solid matter contained in the exhaust gas tends to accumulate in the narrow width of the cavity portion 09. This residual solid matter can obstruct the sliding motion of the flexible thin plate-like blocking member 03 as it curvedly slides within the cavity portion 09.